While using a battery grip with your DSLR or mirrorless camera, you will not be able to power the camera with the FOCUS monitor. The faux battery adapter cable used to supply the camera with power is designed to fit the single battery slot that’s native to each camera body. If you like using battery grips, our suggestion is to continue using the grip to power the camera and use a Sony L Series battery to power the FOCUS monitor.

You need to be careful when attaching a third party power source to your 502 monitor. It has an input range of 6.2V to 8.4V if you exceed 10v you can damage your monitor.
Here is a good dtap to lp-e6 option that you can use with your 502 monitor: http://store.smallhd.com/store/500-accessories/DCA5-KIT